1. Field of the Invention
The present invention relates to a semiconductor light emitting element, and more specifically to a semiconductor laser diode so controlled as to minimize a change of its light emission characteristics according to temperature. Further, the present invention relates to an optical fiber transmission system using the same semiconductor laser diode.
2. Description of the Prior Art
In semiconductor light emitting element, in particular in semiconductor laser, there often exists a problem related to its temperature characteristics. For instance, the threshold current (I.sub.th) of diode lasers is sensitive to change in temperature. Here, since the semiconductor laser functions as a laser only when the driving current exceeds this threshold current (I.sub.th), the temperature fluctuations of the threshold current is one of the important characteristics to be controlled.
In the field of optical communications, the application range has been extended from the state of the art ultrahigh speed optical communication systems for long distance and large capacity, through telecommunication systems of med-range, to data links for short distance. In these optical communication systems, the semiconductor lasers are used as a light source. In LED (light emitting diode), the spontaneous emission below the threshold current is used as the output. Because of a problem that the response speed is slow, the LED cannot be used for the high speed and large capacity communications. In contrast with this, the semiconductor laser has such an advantage that its response speed is much higher. However, semiconductor lasers have a problem related to the threshold current sensitivity to temperature. That is, the system using the semiconductor laser becomes relatively expensive in comparison with the LED. An APC (automatic power control) circuit, for instance is conventionally required to adjust the bias current responding to change of the threshold current. Further, in the case of a DFB (distributed feedback) type laser which can oscillate in single longitudinal mode, a thermoelectric cooler is needed for temperature control, with complicated external circuits.
In the age of multi-media, under such background that there exist explosive needs of the semiconductor lasers in the field of image data communications between optical subscriber systems and between personal computer systems, a semiconductor laser excellent in temperature characteristics and low in cost has been demanded more and more. Here, since the standard of the service range of temperature is so far -40.degree. C. to 85.degree. C., the less fluctuations in characteristics of the threshold current and the optical output (or SE: slope efficiency) are, the better will be the semiconductor laser.
In the semiconductor laser, the optical output P.sub.0 (W) from a facet at the operating current I can be expressed as follows: EQU P.sub.0 =(.eta..sub.d /2)(h.nu./q)(I-.sub.th)(W)
where .nu..sub.d : external differential quantum efficiency
h: Planck constant PA1 .nu.: photon frequency PA1 q: electron charge PA1 I.sub.th : threshold current
Further, the slope efficiency SE can be expressed as SE=increment of the optical output (.DELTA.Po)/increment of the current (.DELTA.I). If current vs. light output relation is linear, SE is equal to (.nu..sub.d /2)(h.nu./q).
Temperature characteristics of a prior art semiconductor laser will be described hereinbelow with reference to FIGS. 7 to 9, in which an InGaAsP/InP laser used for optical communications is shown by way of example.
FIG. 7 is a cross-sectional view showing an FP (Fabry-Perot) type laser using an InGaAsP/InP semiconductor, which is taken along an axial direction thereof. This semiconductor light emitting element can be formed as follows:
First, on an N-type InP semiconductor substrate 1, a multiple quantum well (MQW) active layer 2 of InGaAsP crystal is grown. On this MQW active layer 2, a p-InP layer 4 and a p-InGaAsP ohmic contact layer 5 are grown successively. After that, a buried-hetero (BH) type waveguide structure is formed by mesa-etching the active layer 2 into a stripe shape and further by forming current blocking regions on either side of the stripe-shaped active layer 2. FIG. 9 is a perspective view showing this waveguide structure, in which a mesa structure is formed into a stripe shape on the first principal plane of the semiconductor substrate 1. On the top surface of the mesa portion, a stripe-shaped active layer 2 is formed. Further, a p-InP buried layer 6 and an n-InP burying layer 7 are grown successively on the bottom portion on either side of the active layer 2. Here, since the current is blocked by the reverse junction at the boundary between the burying layers 6 and 7, the current flows only through the stripe-shaped active layer 2.
The electrode 20 is formed on the semiconductor layer 5 formed on the first principal plane of the semiconductor substrate 1. The electrode 21 is formed on the second principal plane of the same semiconductor substrate 1. Further, the surface (opposite side to the optical output side) on the rear facet is coated with a highly reflective film structure, in order to reduce the threshold current and to increase the slope efficiency of the light outputted from the front side.
FIG. 8 is a cross-sectional view showing a DFB type semiconductor laser using an InGaAsP/InP semiconductor. In this DFB type laser, on an n-type InP semiconductor substrate 1, an InGaAsP-MQW active layer 2 and an InGaAsP waveguide layer 3 having a band gap larger than that of the active layer 2 are laminated. Further, a grating 15 is formed on the waveguide layer 3. This grating 15 serves to allow the semiconductor laser to be advantageously feedbacked with the longitudinal mode corresponding to the period of the grating itself, so that the oscillation can be achieved easily in the single longitudinal mode. A grating phase shift 16 which corresponds to .lambda./4 (.lambda.: wavelength in the waveguide) is formed at just a middle portion of the resonator of the grating 15. Owing to the presence of this phase shift 16, the oscillation in the single longitudinal mode can be further facilitated. Further, in the case of the .lambda./4 phase shift DFB type laser, an AR (anti-reflection) coat 31 is formed on both end facets thereof, to suppress the unnecessary FP mode. Further, a p-InP layer 4 and a p-InGaAsP ohmic contact layer 5 are grown on the grating 15 and the phase shift 16. After that, the semiconductor substrate is mesa-etched into a stripe shape, to produce a BH type waveguide structure the same as shown in FIG. 9. Further, two current injecting electrodes 20 and 21 are formed on the semiconductor layer 5 formed on the first principal plane of the semiconductor substrate 1 and the second principal plane of the same semiconductor substrate 1, respectively.
The above-mentioned two prior art semiconductor light emitting elements have some problems as follows:
(1) Temperature dependence of both threshold current and slope efficiency
FIG. 10 shows the temperature dependence of the current versus optical output characteristics of the FP type InGaAsP/InP laser having the MQW active layer 2 so designed as to be oscillated at 1.3 .mu.m wavelength band. As shown in FIG. 10, the threshold current (I.sub.th) of 5 mA at room temperature increases up to 12 mA at 8520 C. Further, the slope efficiency (optical output) decreases by 20-30%. FIG. 10 indicates that the temperature dependence of the optical output at a constant bias current larger than the threshold current becomes further violent, because of the change of the threshold current. Therefore, it is difficult to realize a laser device which does not need any APC circuit.
(2) Temperature characteristics of DFB type laser In the case of the DFB type laser, there exists the case where the threshold current rises due to spatial hole burning in addition to the above-mentioned change. The axial hole burning occurs due to the non-uniformity of photon density along the axial direction thereof.
Once a deviation occurs in photon density, the carrier density distribution also deviates according to the photon density deviation. The change of carrier density causes the change of refractive index of the active layer due to the plasma effect, for instance. In other words, the change of refractive index results in the change of the waveguide structure itself of the DFB laser. Therefore, the phase condition of the laser oscillation changes and thereby the longitudinal mode changes continuously. In the case of the .lambda./4 phase shift DFB type laser, there exists such a tendency that the light is extremely concentrated at the middle of the resonator, that is, at the .lambda./4 phase shift position. FIG. 11 shows an example of the axial distributions of both the photon density S(z, t) and the carrier density N(t) at .kappa.L=2, where .kappa. denotes the coupling factor and L denotes the resonator length. Further, .kappa. depends upon the grating depth and represents a quantity indicative of a light feedback amount of the grating.
The hole burning depends upon the distribution of the density, so that the larger the absolute values of the photon density and the carrier density are, the stronger will be the influence thereof upon the hole burning. For instance, when temperature rises, the threshold carrier density increases, so that the hole burning increases to that extent. In other words, the refractive index changes in such a way as to further cancel out the effect of the .lambda./4 phase shift. Besides, since of the .lambda./4 phase shift makes the phase condition on which the threshold current is minimized, an offset of the phase shift away from this .lambda./4 causes a further increase of the threshold current. Therefore, in the DFB type laser, there exists a tendency that the temperature characteristics further deteriorate due to the vicious cycle caused by the hole burning.